Influence of zinc on ferrous iron bio-oxidation: Biological or physical nature?

Abstract

Ferrous iron bio-oxidation is negatively affected by the presence of heavy metals. Although the available information relates this phenomenon to purely biological aspects, it is contradictory with respect to tolerance levels, mechanisms and kinetics. This dispersion of results may be due to the empirical nature of the approaches which are based on batch cultures and fail to consider the conditions of aeration of the biomass. In the present work, the influence of Zn 2+ in the range of 0–40 g/L is tested in continuous packed-bed bioreactors, by studying oxygen partial pressure and aeration flow rate as variables. Results show that when oxygen is the limiting reagent under identical aeration conditions, the bio-oxidation rate decreases by 0.8% per gram per litre of Zn 2+. The cause of this result is purely thermodynamic; the solubility of oxygen in the medium decreases in equivalent proportions of the bio-oxidation rate owing to the salting-out effect. This finding leads to redesign of reactors for continuous ferrous iron bio-oxidation with the presence of Zn 2+, whereby special attention is paid to the aeration system and its control during the operation.